The present invention relates to a pneumatic tire more particularly to a bead structure for heavy duty tires capable of improving the bead durability.
When a pneumatic tire is loaded, as the bead portion and sidewall lower portion are forced toward the axially outside of the tire, shear stress occurs between the reinforcing cords and the axially outwardly adjacent rubber.
In particular, in the case of tires for heavy duty vehicles such as trucks, buses and the like, as the tires are used under severe service conditions, the shear stress becomes large to increase the likelihood that a separation failure is caused between the reinforcing cords and rubber.
Conventionally, the heavy duty tires are provided with a stiff bead structure to reduce the deformation due to heavy loads for example as shown in Japanese Patent Application Publication No. 2002-205508. More specifically, the bead portion is as shown in FIG. 4, provided between a carcass main portion (a1) and a turned up portion (a2) with a bead apex (b) composed of a bulky stiffener (b1) made of a high modulus rubber and a softener (b2) made of a softer low modulus rubber. In this structure, if it is desired to further decrease the deformation, it is necessary to increase the volume of high modulus rubber forming the stiffener (b1). If the volume is increased, however, the weight is increased and the heat generation during running is also increased. As a result, the bead durability is rather deteriorated, and the fuel consumption of the vehicle is increased, going against to the trend.